From the advent of the telephone, people and businesses have craved communication technology and its ability to transport information in various formats, e.g., voice, image, etc., over long distances. Typical of innovations in communication technology, recent developments have provided enhanced communications capabilities in terms of the speed at which data can be transferred, as well as the overall amount of data being transferred. As these capabilities improve, new content delivery vehicles, e.g., the Internet, wireless telephony, etc., drive the provision of new services, e.g., purchasing items remotely over the Internet, receiving stock quotes using wireless short messaging service (SMS) capabilities etc., which in turn fuels demand for additional communications capabilities and innovation.
Recently, optical communications have come to the forefront as a next generation communication technology. Advances in optical fibers over which optical data signals can be transmitted, as well as techniques for efficiently using the bandwidth available on such fibers, such as wavelength division multiplexing (WDM), have resulted in optical technologies being the technology of choice for state-of-the-art long haul communication systems.
Depending upon the relative locations of the data source and the intended recipient, optical data signals may traverse different optical communication systems in their path between the two locations, e.g., for trans-Atlantic data connections. For example, optical signals may traverse both a terrestrial optical communication system and a submarine optical communication system. As shown in FIG. 1, a terrestrial signal is processed in a WDM terminal 12 of a submarine optical communication system 10 for transmission via optical fiber 14. For long haul optical communications, e.g., greater than several hundred kilometers, the optical signal is periodically amplified to compensate for the tendency of the data signal to attenuate. Therefore, in the submarine system 10, line units 16 amplify the transmitted signal so that it arrives at WDM terminal 18 with sufficient signal strength (and quality) to be successfully transformed back into a terrestrial signal.
Conventionally, erbium-doped fiber amplifiers (EDFAs) have been used for amplification in the line units 16 of such systems. As seen in FIG. 2(a), an EDFA employs a length of erbium-doped fiber 20 inserted between the spans of conventional fiber 22. A pump laser 24 injects a pumping signal having a wavelength of, for example, approximately 1480 nm into the erbium-doped fiber 20 via a coupler 26. This pumping signal interacts with the f-shell of the erbium atoms to stimulate energy emissions that amplify the incoming optical data signal, which has a wavelength of, for example, about 1550 nm. One drawback of EDFA amplification techniques is the relatively narrow bandwidth within which this form of resonant amplification occurs, i.e., the so-called erbium spectrum. Future generation systems will likely require wider bandwidths than that available from EDFA amplification in order to increase the number of channels (wavelengths) available on each fiber, thereby increasing system capacity.
Distributed Raman amplification is one amplification scheme that can provide a broad and relatively flat gain profile over a wider wavelength range than that which has conventionally been used in optical communication systems employing EDFA amplification techniques. Raman amplifiers employ a phenomenon known as “stimulated Raman scattering” to amplify the transmitted optical signal. In stimulated Raman scattering, as shown in FIG. 2(b), radiation from a pump laser 24 interacts with a gain medium 22 through which the optical transmission signal passes to transfer power to that optical transmission signal. One of the benefits of Raman amplification is that the gain medium can be the optical fiber 22 itself, i.e., doping of the gain material with a rare-earth element is not required as in EDFA techniques. The wavelength of the pump laser 24 is selected such that the vibration energy generated by the pump laser beam's interaction with the gain medium 22 is transferred to the transmitted optical signal in a particular wavelength range, which range establishes the gain profile of the pump laser.
Regardless of the amplification technique(s) employed, mechanisms are provided for, e.g., organizing the information in the optical signals, providing capacity management and activating backup protection schemes, typically by way of a protocol layer, e.g., Synchronous Optical Network (SONET). Among other things, SONET standardizes the rates at which data streams are transmitted via optical data signals using the Optical Carrier (OC) system. For example, an OC48 data stream carries data at a rate of about 2.5 Gb/s (including overhead) and an OC192 data stream carries data at a rate of about 10 Gb/s (again including overhead). Many of today's WDM optical communication systems transmit OC192 data streams over each channel.
To prepare terrestrial optical signals for transmission over a submarine optical network, or to prepare submarine optical signals for transmission over a terrestrial network, an interface between the terrestrial optical signals and the submarine optical signals is typically provided. Today's interfaces typically provide for the transmission of terrestrial optical signals over a submarine optical system using OC192 data streams. As development of optical systems progresses, and demand for bandwidth increases, it is envisioned that the communications market may progress from common usage of 10 Gb/s data streams (OC192) to common usage of 40 Gb/s data streams (OC768). During the transition period, it is likely that both data rates will be commonplace in optical communication systems.
Accordingly, it would be desirable to provide an interface between optical communication systems, e.g., terrestrial and submarine, which is capable of handling different data rates, in particular OC192 and OC768. Moreover, it would be desirable that such interfaces be relatively transparent to the systems being interfaced and use common components for ease of manufacture and cost savings.